Late blight is the main disease of potatoes around the world. Because of the difficulties in applying effective control methods, the introduction of resistant cultivars represents a safe strategy. The potato species Solanum tarijense represents an attractive resistance source as its adaptation to long days is promissory, producing tubers of good size and aspect. Deposition of structural compounds like lignin and callose were described as a non-specific resistance mechanism. In this work, we measured polyphenoloxidases (PPO) and peroxidases (POX) activities and the accumulation of phenols, lignin and callose and their correlation with the resistance levels of S. tarijense. Clones Oka 6320.9 and Oka 5632.11 showed low infection rates and these were correlated with a higher accumulation of phenols, lignin and callose and a strong induction of PPO and POX activities. However, in highly infected clones, a lower or no accumulation of these compounds was observed. These results demonstrate a correlation between the amount of defence molecules and the resistance levels according to the detached-leaf assay. However, more field experiments are required to validate these results.

Potato leaves infected with Phytophthora infestans produced a serine protease inhibitor (PLPKI) with specificity for microbial proteases. Sequencing of the first twenty residues at the NH 2 -terminus of the mature PLPKI polypeptide demonstrated that PLPKI is a novel member of the potato protease inhibitor I family. PLPKI inhibited the activity of extracellular proteases produced by two pathogens of potato, P. infestans and Rhizoctonia solani, but was inactive against proteases secreted into the culture media by the binucleate Rhizoctonia N2, a non-pathogenic fungus for potato. Western blot analysis showed a positive correlation between the levels of PLPKI and the degree of horizontal resistance, showing its highest accumulation in clone OKA 5632.11, which has been described as highly resistant. This correlation, together with the ability of PLPKI to completely abolish the secreted serine protease activity of P. infestans, suggests that PLPKI may have an active role in protecting potato plants from this pathogenic oomycete and that it could be used as a suitable biochemical marker to help breeders in the selection of cultivars with high degree of horizontal resistance.